1. Field of the Invention
The invention relates to a piezoelectric actuator, for instance for actuating a mechanical component such as a valve or the like.
2. Description of the Prior Art
It is widely known that by using what is known as the piezoelectric effect, a piezoelectric element may be constructed from a material that has a suitable crystal structure. When an external electrical voltage is applied to these piezoelectric and electrostrictive ceramics, a mechanical reaction of the piezoelectric element ensues, which as a function of the crystal structure and the regions where the electrical voltage is applied represents a pressure or tension in a predeterminable direction.
Because of the extremely fast and precisely regulatable reciprocation effect, such piezoelectric actuators may be contemplated for constructing final control elements, for instance for driving switching valves in fuel injection systems in motor vehicles. The voltage- or charge-controlled deflection of the piezoelectric actuator is utilized for positioning a control valve, which in turn regulates the stroke of a nozzle needle.
Since the required electrical field intensities for actuating the piezoelectric actuator are in the range of several kV/mm, and as a rule moderate electrical voltages for triggering are desired, the construction of this piezoelectric actuator is done here in multiple layers of metallized piezoceramics, stacked one above the other, to form what is known as a multilayer actuator. To that end, between each of the layers there are inner electrodes, applied for instance by a printing process, and there are also outer electrodes by way of which the electrical voltage is applied. One typical method for producing such layers is film casting. To produce the inner electrodes, the individual layers are metallized and stacked one above the other, and then between two layers, with inner electrodes of different polarity, the piezoelectric effect develops.
At the top and bottom regions, however, the inner electrodes as a rule are missing, since on the one hand, a certain insulation distance is needed at the end faces, to avoid short circuits toward the outside, and on the other, passive zones are utilized for electrical connection of the outer electrodes. Passive regions may also be present inside the actuator. Often, however, in the zone between the region with inner electrodes, as an active zone, and the region without inner electrodes, as a passive zone, there is the risk of cracks. This phenomenon shortens the service life and can lead to the total failure of the actuator.
It is known per se from German Patent Disclosure DE 100 25 998 A1 that passive regions of variable length are disposed on both ends of the ply or layer construction. A total length of the piezoelectric actuator fixedly defined for a particular installation situation is attained here by way of reducing the length of the piezoelectrically inactive top and/or bottom piece, for instance by hard machining by means of grinding or the like.
In this known piezoelectric actuator, on the one hand, the passive layers may be of the same ceramic material as the active region, but with outer electrodes that are contacted electrically on only one side or not at all, so that the inactive regions are also penetrated by the metal layers of the inner electrodes. On the other hand, the respective inactive region may also be a completely electrically insulated metal or ceramic block, which can simply be glued to the piezoelectrically active region, for instance.